1. Field of the Invention
The present invention relates to a semiconductor device and a method for manufacturing the semiconductor device.
In this specification, a semiconductor device means a general device which can function by utilizing semiconductor characteristics, and an electro-optic device, a semiconductor circuit, and an electronic device are all semiconductor devices.
2. Description of the Related Art
A technique by which transistors are formed using semiconductor thin films over a substrate having an insulating surface has been attracting attention. Such transistors are applied to a wide range of electronic devices such as integrated circuits (ICs) or image display devices (display devices). A silicon-based semiconductor material is widely known as a material for a semiconductor thin film applicable to a transistor. As another material, an oxide semiconductor has been attracting attention.
For example, a transistor whose active layer includes an amorphous oxide containing indium (In), gallium (Ga), and zinc (Zn) and having an electron carrier concentration of less than 1018/cm3 is disclosed (see Patent Document 1).
A transistor including an oxide semiconductor is known to have a problem of low reliability because of high possibility of fluctuation in electric characteristics, although the transistor including an oxide semiconductor can operate at higher speed than a transistor including amorphous silicon and can be manufactured more easily than a transistor including polycrystalline silicon. For example, the threshold voltage of the transistor fluctuates after a bias-temperature stress test (BT test). Note that in this specification, a threshold voltage refers to a gate voltage which is needed to turn on a transistor. A gate voltage refers to a potential difference between a source electrode and a gate electrode when the potential of the source electrode is used as a reference potential.